Inflatable packers can be activated in a number of ways, including by movement of a control string upon which the packers are lowered into a bore hole/well, and by inflation lines run within the annular space of the well bore. In the former configuration mechanical manipulation of the packers permits changes in their mode of operation and is suitable for deeper and higher pressure operation. In the latter configuration typically nitrogen gas is used for inflating the packers and is suitable for shallow and low pressure operation.
One of the challenges with using liquid as an inflation medium, is that in unsaturated (dry) formations there is an overbalance of hydrostatic pressures between the liquid in the inflation line versus the hydrostatic pressure present in the annulus (annular space). This overbalance in hydrostatic pressure can result in the packer prematurely inflating and ultimately will not allow the packer to deflate, typically by venting to surface. Ordinarily this is overcome by using gas inflation systems. However, this requires a supply of nitrogen gas. Also, gas packers have a limited operating depth due to the limits of pressure found in compressed gas cylinders. Gas packers are also prone to inaccuracies in their operation and hence liquid controlled inflatable packers are preferred.
In order to use liquid operated packers in boreholes with low static water levels we found it most effective to provide a dump valve to isolate the hydrostatic pressure in the inflation line from the packer and at the same time provide communication from the inflatable packer to the annulus to allow downhole deflation.
We found that the hydraulic dump valve prevents premature inflation by providing resistance to the overbalance of hydrostatic pressure in the inflation line, which would otherwise inflate the packer (prematurely). The hydraulic dump valve also provides a flow path from the inflatable packer to the annulus, therefore as long as the hydraulic dump valve is able to resist the overbalance of hydrostatic pressure in the inflation line, the packer will deflate/vent liquid downhole, which eliminates the effects differential hydrostatic pressure present in unsaturated sections of the borehole or boreholes with low static water levels.
The hydraulic dump valve of the present invention prevents premature inflation and enables downhole deflation. It allows a liquid filled inflatable packer to be reliably operated (inflate/deflate) in boreholes which are unsaturated or have a low static water level.